Topic 3.3 · Core and Supplement
Active Transport
Diffusion can only ever move particles down a concentration gradient. Sometimes a cell needs the opposite — to move a substance from where it is scarce to where it is already abundant — and that is a job diffusion simply cannot do. Active transport is the process that can, and the reason it can is the entire point of this topic.
Active transport
The full Core definition has three parts, and Cambridge expects all three:
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration.
- It occurs through a cell membrane. The particles are being moved across the boundary of a cell, not simply through open space.
- Movement is against the concentration gradient.The net movement runs lower concentration → higher concentration — the exact opposite direction from diffusion.
- Energy is required. That energy is supplied by respiration, and this requirement is what fundamentally separates active transport from diffusion and osmosis, neither of which needs the cell to spend any energy at all.
Why active transport is necessary — Supplement
Imagine a cell needs a particular mineral ion, but that ion is already at a higher concentration inside the cell than outside it. Diffusion cannot produce any further net movement into the cell in that situation, because diffusion only ever moves particles down a concentration gradient, and the gradient here runs the wrong way for the cell’s needs.
Active transport solves exactly this problem. By spending energy from respiration, a cell can go on taking up molecules or ions even when doing so means moving them against their concentration gradient — something no amount of waiting for diffusion to catch up would ever achieve. This is precisely why Cambridge requires Supplement candidates to understand the importance of active transport, not just its definition: without it, a cell would be limited to only ever gaining substances that happen to already be more concentrated outside it.
Active transport in root hair cells — Supplement
Root hair cells are the specific biological example named in the Cambridge syllabus, and they make the problem concrete. Plants need mineral ions from the soil, but the concentration of a particular mineral ion in the soil around a root is often lower than its concentration already inside a root hair cell. Movement from soil into the cell would therefore have to run:
lower concentration → higher concentration
which diffusion is simply unable to cause. Instead, the root hair cell moves those mineral ions in by active transport, spending energy from respiration to do it. This is what allows a plant to obtain the mineral ions it needs even when the soil around its roots contains relatively little of them — a plant is not passively waiting for a favourable concentration gradient to appear; it is actively working to create the uptake it needs. The wider structure and function of root hairs, and how this uptake connects to water movement through the whole plant, is covered in more depth in the transport-in-plants chapter.
Protein carriers — Supplement
Active transport does not happen at just any point on the cell membrane — it involves specific protein carriers embedded in the membrane, which physically move molecules or ions across during the process. For this syllabus, the relationship worth holding in your head is:
particle + membrane carrier protein + energy from respiration → transport against the concentration gradient
This does not replace the definition of active transport — it sits alongside it. The definition tells you where particles move and why energy is needed; the carrier proteins explain how the membrane itself participates in getting them there. A full Supplement answer on active transport usually needs both halves: the direction-and-energy definition, and the fact that carrier proteins are what actually do the moving.
Diffusion, osmosis and active transport — protein carriers compared
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Protein carriers in the 0610 requirement | Not specified | Not specified | Protein carriers move molecules or ions across the membrane — Supplement |
That row is worth reading carefully: Cambridge does not require you to say diffusion and osmosis never involve membrane proteins in any circumstance — only that, for this syllabus, protein carriers are explicitly named as required knowledge specifically for active transport at Supplement level. The full comparison of all three processes side by side — what moves, in which direction, and whether a membrane and energy are involved — is on the chapter hub.